Semiconductor devices and liquid crystal display devices are manufactured by a procedure known as photolithography, in which a pattern formed on a mask is transferred onto a photosensitive substrate. An exposure apparatus used in this photolithography process includes a mask stage which supports the mask and a substrate stage which supports the substrate, and transfers the pattern on the mask via a projection optical system while constantly moving the mask stage and the substrate stage. Recently, in order to deal with increasing integration of device patterns, there is a demand to further increase the resolution of projection optical devices. The resolution of a projection optical device increases when the exposure wavelength being used is shorter, and when the projection optical system has a higher numerical aperture. Accordingly, exposure wavelengths used in projection optical systems are becoming shorter, and their numerical aperture is increasing, year by year. While the current mainstream exposure wavelength is the 248-nm wavelength of a KrF excimer laser, an even shorter length, the 193-nm wavelength of an ArF excimer laser, is also going into actual use.
As with the resolution, depth of field (DOF) is also important when exposing light. Resolution R and depth of field δ are expressed by the following equations.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
where λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. As is clear from Equations (1) and (2), when the exposure length λ is shortened and the numerical aperture NA is increased in order to increase the resolution R, the depth of field δ becomes narrower.
If the depth of field δ is too narrow, it becomes difficult to match the substrate surface to the projection face of the projection optical system, and the field margin during the exposure operation may not be sufficient. As an example of a method of actually shortening the exposure wavelength while increasing the depth of field, Patent Document 1 discloses the immersion method. In this immersion method, an area between a bottom face of the projection optical system and the substrate surface is filled with a liquid, such as water or an organic solvent, to form an immersion region, and, utilizing the fact that the wavelength of exposure light in liquid becomes 1/n of its wavelength in air (where n is the refractive index of the liquid, and is normally approximately between 1.2 and 1.6), the resolution is increased while multiplying the depth of field n times.    Patent Document 1: PCT International Publication No. WO 99/49504